1. Feild of the Invention
The present invention refers to a photodetector device with high sensitivity and equipped with an adjustable micrometric diaphragm integrated in the photodetector device itself. More specifically it is a detector device suitable for use in confocal microscopes.
2. Description of the Art
Photodetector devices that enable optical signals to be measured are already known. There are cases in their application in which the information of interest is brought only by the light signal incident on a small well-defined area. In such cases a photodetector is required to generate an output signal only in correspondence with the arrival of the photons on that small area.
In particular in confocal microscopes a light detector apparatus is used which functions in the above mentioned manner and which in addition has a very high detection sensitivity, suitable for working with ultra-weak illumination intensity, in various cases even managing to detect single photons. To reach these high sensitivities it is necessary to use photodetectors, which also with ultra-weak illumination intensity supply electrical signals of level higher than the noise of the electronic circuits that process the signals themselves, so that the sensitivity is not limited by the noise of the circuits. Photodetector devices which are available with these characteristics are photomultiplier tubes (PMT), avalanche photodiodes that work in a similar way to an amplifier (Avalanche PhotoDiodes APD), and avalanche photodiodes that work in the Geiger mode (Single Photon Avalanche Diodes SPAD). The selection of the light that arrives at the photodetector device is obtained with a mechanical micrometric diaphragm with has an accurately defined diameter and position of the opening, placed in front of the photodetector itself.
In various cases it is also required that the area selected be adjustable so as to meet the various needs in the various phases of a same measurement or in a sequence of measurements that are made under different conditions. Typical cases in a confocal microscope are those in which it is necessary to detect the light signal coming from very small samples (also single molecules diluted in a fluid), which is difficult to do by using a very small diameter diaphragm. In these cases, a preliminary observation of the samples is necessary. Such a preliminary observation can be carried out by using a micrometric diaphragm with a larger diameter so as to collect the light from a greater observed volume. When the object being looked for has been identified, a narrower micrometric diaphragm is used so as to obtain a more precise measurement, thereby limiting the observation to a smaller and better defined volume. Nevertheless, this requires the use of an adjustable mechanical micrometric diaphragm, which implies an increase in size, complexity and cost for the detection apparatus, an increase that turns out to be particularly remarkable if the apparatus is made so that it can be controlled by the electronic control system of the microscope.
It is beneficial to avoid the use of electromagnetic actuators and mobile mechanical parts. Instead,it is beneficial to use a photodetector that has a sensitive area whose dimensions can be controlled only by electronic means and that has the required high sensitivity.
A solution to this problem can be found in the use of a photodetector which is equipped with a sensitive area divided into small parts (pixel), that is, an array detector or an image detector. Nevertheless, the presently available array detectors and the image detectors have characteristics which are not very suitable for the solution of the above-mentioned problem.
Among the photomultiplier tubes (PMT) in industrial production, types with the anode being subdivided into small areas are available, but these areas are not small enough and are separated by sizeable dead spaces, which significantly reduce the detection efficiency. In addition, these PMT have a high number of separate electrical outputs (one per pixel), which increase the complexity, overall dimension and cost of the electronic circuits for processing the signals.
Other types of PMT permit a detection of the optical impulses that are sensitive to the position of incidence within a detection area that is continuous, that is, without dead spaces. Nevertheless, such types of PMT are costly and cumbersome and require the use of complex electronic circuits for extracting the information concerning the position of incidence of the optical signal inside the sensitive area. These types of PMT can work at a high sensitivity level, even at single-photon detection level, but the maximum allowable counting rate of photons that are detected on the whole area is less than that reached by an ordinary PMT. This limitation reduces considerably the dynamic range of the measurement.
The APD arrays, which are also presently available from industrial production, show drawbacks similar to those of the above-mentioned PMT with segmented anode, to which it must be added that of having a multiplication gain that is not high (values from a few tens to a few hundreds), which is not uniform for the various pixels and which varies as the temperature varies.
The SPAD arrays, which in contrast to the above-mentioned detectors are not as yet available commercially and are a research objective, present the difficult problem of the optical cross-talk between pixels. This cross-talk is due to the optical emission by the avalanche current charge carriers in a pixel, which generates false photon detection signals in the adjacent pixels. In order to eliminate the optical cross-talk between the pixels, an efficient optical insulation of the pixels must be provided, but this present considerable technological manufacturing difficulties and also causes an increase of the dead spaces between the pixels and of the cost of production of the SPAD arrays.
From the U.S. Pat. No. 5,900,949, it appears that also CCD image detectors (Charge coupled devices) have been used for the stated purpose. These detectors are available from industrial production and have various interesting characteristics (good quantum detection efficiency, flexibility of use, etc.). However the CCD image detectors have no internal gain and therefore their sensitivity is definitely lower and it is not possible to detect single photons with them.